Learn Hardcore Korean: jihacheori beoseuboda deo bissayo. - The subway is more expensive than the bus.

보다 is the comparative particle meaning “than.” In 버스보다, it marks 버스 as the thing you’re comparing to. So 지하철이 버스보다... literally means “The subway, compared to the bus...”

Why is 더 used when 보다 already indicates a comparison?

While -보다 alone can express “than,” adding 더 (“more”) emphasizes the degree: 더 비싸요 = “is more expensive.”

버스보다 비싸요 = “It’s expensive than the bus.” (understood)
버스보다 더 비싸요 = “It’s more expensive than the bus.” (clearer, more natural)

Why do we use 지하철이 here? Could we say 지하철은 instead?

-이/가 marks the subject (what the sentence is about).
-은/는 marks the topic or contrast.
You can say 지하철은 버스보다 더 비싸요, which shifts nuance to “As for the subway, it’s more expensive than the bus.” Both are correct; 지하철이 simply feels like a straightforward subject marker.

How would I invert the sentence to emphasize that the bus is cheaper?

Flip the items and use 싸다 (“to be cheap”):

버스는 지하철보다 더 싸요.
This means “The bus is more cheap than the subway,” i.e. “The bus is cheaper than the subway.”

What happens to 비싸다 when we say 비싸요?

비싸다 is the dictionary (infinitive) form. To make it polite and present-tense, you drop -다 and add -요, giving 비싸요. This is the standard polite informal style in Korean.

Is it possible to omit 더 and just say 지하철이 버스보다 비싸요?

Yes. Koreans often say 지하철이 버스보다 비싸요 without 더, and it’s still perfectly natural. Adding 더 simply strengthens the “more” aspect.

What’s the difference between 보다 and 보다도?

Both mean “than,” but:

보다 is the everyday comparative particle.
보다도 is more formal or literary, often used in writing or very polite speech. In casual conversation, stick with 보다.

Can I use another adjective, like 싸다, to express the same idea?

Yes. If you want to say “The subway is cheaper than the bus,” you’d say:

지하철이 버스보다 더 싸요.
Just switch 비싸다 (“to be expensive”) to 싸다 (“to be cheap”).